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Potential Tropical Cyclone Nine Forecast Discussion Number 6
2020-07-29 22:44:09| National Hurricane Center (Atlantic)
Issued at 500 PM AST Wed Jul 29 2020 000 WTNT44 KNHC 292044 TCDAT4 Potential Tropical Cyclone Nine Discussion Number 6 NWS National Hurricane Center Miami FL AL092020 500 PM AST Wed Jul 29 2020 Deep convection has continued to increase in association with the disturbance since the previous advisory. More recently, a band has developed over the southwestern portion of the broad circulation and it appears that the system may be closer to acquiring a well-defined center. Earlier ASCAT data that arrived shortly after the release of the previous advisory revealed a large area of 35-40 kt winds well to the north of the vorticity maximum. This supports maintaining the initial intensity of 40 kt. The estimated motion remains a brisk 290/20 kt. A strong subtropical ridge that extends westward over the western Atlantic should continue to steer the system west-northwestward over the next couple of days, bringing the system near or over Hispaniola late tonight and Thursday. A mid-latitude trough moving into the central United States on Friday is foreast to weaken the western portion of the ridge which should cause a reduction in the forward speed of the system and a turn toward the northwest and north later in the forecast period. The latest iterations of the dynamical models have trended toward a more eastward solution, taking the system near or just east of the Florida peninsula. Since the system still lacks a well-defined center it is not surprising to see these inconsistencies in the model runs. As a result, the NHC forecast has been shifted eastward, but it remains to the west of the latest multi-model consensus aids out of respect for continuity and the continued possibility of further model shifts. The system is still forecast to become a tropical storm before reaching Hispaniola, and some slight strengthening could occur before landfall Thursday morning. Some weakening is likely as the system moves over that island. The system is likely to take some time to recover after its passage over land, and given the forecast for at least moderate south to southwesterly shear, only gradual strengthening is indicated at that time. As mentioned above, some of the global models are now showing a track farther away from eastern Cuba and east of Florida, and if that occurs the NHC intensity forecast could be somewhat conservative. Interests in the Bahamas, Cuba, and Florida should continue to monitor forecasts as changes to both the track and intensity are likely. Key Messages: 1. Potential Tropical Cyclone Nine will produce heavy rains and potentially life-threatening flash flooding and mudslides across the northern Leeward Islands, the Virgin Islands, Puerto Rico, the Dominican Republic, northern Haiti, and over the Inagua Islands into the southeastern Bahamas. 2. Tropical storm conditions are likely across portions of the Leeward Islands, the Virgin Islands, and Puerto Rico through tonight and will spread westward to portions of the Dominican Republic, Haiti, and the southeastern Bahamas and Turks and Caicos on Thursday and Thursday night. Tropical Storm Warnings are in effect for these areas. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system. 3. The details of the long-range track and intensity forecasts remain more uncertain than usual since the system does not have a well-defined center and it is expected to move near or over portions of the Greater Antilles later this week and move near or over Florida this weekend. While this system could bring some rainfall and wind impacts to portions of Cuba, the central and northwest Bahamas, and Florida later this week and this weekend, it is too soon to determine the location or magnitude of those impacts. Interests there should monitor the progress of this system and updates to the forecast over the next few days. FORECAST POSITIONS AND MAX WINDS INIT 29/2100Z 16.4N 65.6W 40 KT 45 MPH...POTENTIAL TROP CYCLONE 12H 30/0600Z 17.5N 68.1W 45 KT 50 MPH...TROPICAL CYCLONE 24H 30/1800Z 19.2N 71.0W 40 KT 45 MPH...INLAND 36H 31/0600Z 20.9N 74.2W 40 KT 45 MPH...OVER WATER 48H 31/1800Z 22.3N 76.9W 45 KT 50 MPH 60H 01/0600Z 23.6N 78.9W 50 KT 60 MPH 72H 01/1800Z 25.5N 80.8W 45 KT 50 MPH...INLAND 96H 02/1800Z 28.6N 82.1W 40 KT 45 MPH...INLAND 120H 03/1800Z 31.5N 82.5W 35 KT 40 MPH...INLAND $$ Forecaster Brown
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